Rats given thebaine and (+)-salutaridine, intermediates in the biosynthesis of morphine in poppy plants, produced significant levels of codeine and morphine in tissue of rats, suggesting that they may represent intermediates in the mammalian synthesis of morphine. Since both, (+)-salutaridine and (-)-thebaine were as pure as possible, it seems justify to exclude contamination of the two alkaloids with codeine or morphine as the source of mammalian codeine.